1. Field of Invention
The present invention relates generally to the field of mobile and fixed communication systems. More specifically, the present invention is related to optimizing random access channel time vicinity estimation, in direct sequence spread spectrum systems, however the invention could be employed to other modulation technique as well.
2. Discussion of Prior Art
One of the common modes of communication today is through user""s equipment (UE). UEs are operated in two modes, idle mode (or listening mode) and dedicated mode (or sending and receiving mode). UEs in dedicated mode monitor the surrounding base stations for handover in mobile case and other information. UE communications commonly utilize two kinds of channels, traffic channels (TCH) and control channels. Traffic channels (TCHs) are used to carry speech and data traffic, whereas control channels are used (by idle mode mobiles) to exchange signaling information or change to dedicated modes. Some examples of common control channels include:
Broadcast Control Channel (BCCH): serves for base station identification, broadcasts, and frequency allocations.
Frequency Correction Channel (FCCH) and Synchronization Channel (SCH): used for synchronization, and physical layer definition (time slots, burst time . . . ).
Random Access Channel (RACH): used by mobile to request access to the network.
Paging Channel (PCH): used for locating the mobile user.
Access Grant Channel: (AGCH) used to obtain a dedicated channel. (Following the request of RACH.)
Of growing interest is the Random Access CHannel or RACH, which is defined for transmission, as depicted in FIG. 1, from a plurality of UEs 102, 104, 106, 108, 110, 112, 114 to the base-station (BS) 100. FIG. 2 describes the general RACH structure. RACH is comprised of, a preamble part 202, 204, 206, 210 and a message part 208, 212. FIG. 2a describes one of the RACH structures wherein RACH message 208 is transmitted adjacent to the preamble 202, 204, 206 that was granted by the BS 100. Another RACH structure is detailed in FIG. 2b, wherein the RACH message part 212 is always transmitted after the preamble 210 (and hence no grant from BS 100 is needed). As can be seen from FIG. 3, the BS frame timing being seen by each of the terminals is different (T0, T1, and T2) based on disparate propagation delays between each of the terminals and the BS 100. The difference in propagation delay is caused by the terminals"" deployment (as is the case for fixed and Mobile deployment/distribution) and multi-path physical phenomenon. Therefore, the received preambles of some terminals at the BS 100 aren""t synchronized to any specific timing, nor is there any synchronization among them. Thus, the BS 100 must xe2x80x98searchxe2x80x99, as shown in FIG. 3, for the start of the preambles at an expected time window W 302. Receiving of terminal RACH is a very intensive task and one of the important things is the time vicinity that the receiver expects to get such an access; since in that time the receiver can use the resources in a better way. There are a variety of ways that are described in the prior art for determining the starting point of the preambles.
One of the ways to determine the starting point of the preambles is to estimate the receiving power to get a better starting point for the receiver. However, such an approach doesn""t work for direct sequence (DS) spread spectrum or DSSS, since the terminal""s channel share the same bandwidth (BW) at the same time. DSSS is probably the most widely recognized form of spread spectrum (and is commonly used in cellular communications) wherein first, the pseudo-noise (PN) code is modulated onto the information signal using one of several modulation techniques (for example BPSK or QPSK). Next, a doubly balanced mixer is used to multiply the RF carrier and PN modulated information signal. Thus, the RF signal is replaced with a very wide bandwidth signal and has the spectral equivalent of a noise signal. The use of DSSS is advantageous in communication systems as it is utilized effectively to conserve the bandwidth, to increase the data rate, and to increase immunity to noise interference.
In a DSSS scenario, as described above, there is a need for a system that estimates the RACH start time. The following references describe prior art in the field of random access channels, but none provide for the present invention""s method of estimating the RACH starting time.
U.S. Pat. No. 4,979,168 assigned to U.S. Philips Corporation, provides for a Dynamic Sensing Point CSMA Packet Switching Controller. Disclosed is an estimation method for optimization of the sensing time interval. The reference teaches narrowing of a sensing interval by estimation using probabilistic techniques.
U.S. Pat. No. 5,822,359, assigned to Motorola, Inc., provides for a Coherent Random Access Channel in a Spread-spectrum Communication System and Method. A known synchronization is correlated with a received communication signal to generate a correlation peak when a synchronization message is present. A channel response is determined from the correlation peak and is revised based on estimates derived from the stream of reference samples.
U.S. Pat. No. 5,850,392, assigned to Ericsson Inc., provides for Spread Spectrum Random Access Systems and Methods for Time Division Multiple Access Radiotelephone Communication Systems. In communicating the spread spectrum random access channel signal, a random access channel signal, representing the random access channel message, is direct sequence modulated according to the spreading sequence to produce a direct sequence modulated random access channel signal. A synchronization sequence may be associated with a plurality of spreading sequences. The synchronization sequence may be first detected from the communicated spread spectrum random access channel signal, and in response to detection of the synchronization sequence, one of the plurality of spreading sequences associated with the detected synchronization sequence may be detected.
U.S. Pat. No. 5,883,887, assigned to Mitsubishi Denki Kabushiki Kaisha, provides for a Radio Data Transmission System. Of interest is the method disclosed to sense a RACH access including the probability of RACH shifting. However, the reference fails to disclose the preamble detection method of the present invention.
In all the above-described systems there is no mention of estimating RACH start times as mentioned earlier. Whatever the precise merits, features and advantages of the above cited references, none of them achieve or fulfills the purposes of the present invention. The current invention relates to the RACH Starting Time Vicinity Estimation (STVE) and is further compliant with the 3rd generation for cellular systems. The present invention also enables to synchronize on the timing and minimize the uncertainty of the preamble phase at the BS, by which the complexity of the RACH receiver is significantly reduced. These and other objects are achieved by the detailed description that follows.
Estimating a starting time vicinity associated with a random access channel (RACH) wherein a terminal transmission method and a received signal model are utilized.
A first method, called In*Qn RACH starting time vicinity estimation (STVE), utilizes a signal that is obtained by multiplying the In and Qn branches of a sample. Exact starting time of a preamble signal is calculated from the peak output of the phase metrics expression (for both even and odd periodicity). A second method, called (In+jQn)2 RACH STVE, performs a square operation on the received complex signal and calculates the peak output of the corresponding phase metric to extract the exact starting time of a preamble signal. The present invention, in one embodiment, is in compliance with the standard of the 3rd generation for cellular systems whereby the timing is synchronized, the uncertainty of the preamble phase (at the base station) is minimized, and the complexity of the RACH receiver is significantly reduced.